The invention relates to the field of analog-to-digital converters (ADCs), and in particular to determining and compensating for a characteristic of an analog-to-digital converter (ADC).
An ADC converts an analog input voltage with a generally constant sampling frequency digital output signal. ADCs have the disadvantage of not having an ideal characteristic. Departures from such an ideal characteristic are often termed differential non-linearity (DNL) or integral non-linearity (INL), where DNL denotes a maximum step width error and INL denotes an error between a quantized value and an ideal continuous value.
To reduce these errors, for a known characteristic of the ADC, it is known to add a voltage-dependent correction value to the digital data word. The correction value comes, for example, from a table in which all the voltage-dependent correction values are kept.
The characteristic can be measured by applying a defined and known signal. Such a test signal is usually a voltage ramp, a triangle, or a sinusoidal signal. A compensation circuit designed by Temerinac et. al. measures an ADC by a sinusoidal signal, which is compared at the digital side with a reconstructed ideal sine wave. Parameters for amplitude, phase and d.c. voltage offset of this reconstructed sine are obtained from the converted signal by a phase locked loop (PLL). From the voltage-dependent differences between the converted and the reconstructed signal, the values are formed as coefficients for the correction table. During the functioning of the ADC, these coefficients are subtracted from the input signal of the ADC.
Such a technique is based on a static characteristic without a memory. However, if the ADC has different states depending on the past history of the signal, a static compensation of the characteristic is not sufficient, or not even possible. Instead, one needs to set up and appropriately apply a different set of correction coefficients for the signal, i.e., for each signal history. But this leads to a number of (n+1)-dimensional coefficient fields with n as the number of past sampling values to be considered.
There have been efforts to mathematically describe by models ADCs having memory (Volterra, Wiener), for example, in John Tsimbinos, “Identification And Compensation of Non-Linear Distortion, The Levels,” Australia 1995, but such models lead to elaborate computations.
There is a need for simplified error compensation for an ADC with memory.